Urethane prepolymers have been modified in the past to improve or add to their basic properties by end-capping some or all of the isocyanate groups with various organofunctional silanes. Among these methods, U.S. Pat. No. 3,632,557 teaches the complete end-capping of conventional polyurethane prepolymers with primary and secondary aliphatic aminosilanes. U.S. Pat. No. 3,979,344 details a room temperature curable silicon terminated organic sealant composition comprising a small quantity of 3-(N-2-aminoethyl)aminopropyltrimethoxysilane end-capper to improve the sealant's cure speed. U.S. Pat. No. 4,222,925 details the same composition as is given in U.S. Pat. No. 3,979,344 but also incorporates a reinforcing carbon black filler. Sealants having improved elongation and flexibility were prepared using silane end-capped polyurethane polymers obtained from silane monomers having at least one dialkoxy group and an organofunctional group with at least one active hydrogen atom in U.S. Pat. No. 4,645,816.
However, polyurethane prepolymers end-capped with primary aminofunctional silanes contain an active hydrogen atom that is capable of further reactions with the isocyanate end groups. This reactivity can lead to undesirable stability of both the polymer and sealant compositions. Several methods teach the use of secondary aminofunctional silane end-cappers. For example, U.S. Pat. No. 4,374,237 teaches the curable isocyanate terminated polyurethane prepolymers of at least part of whose terminal isocyanate groups have been reacted with a secondary amine containing silane monomer having two trialkoxy silane groups. U.S. Pat. No. 4,474,933 describes crosslinkable polyurethane resin mixtures that have been end-capped with various primary and secondary difunctional aminosilanes. More recently, U.S. Pat. No. 5,364,955 teaches the use of N-alkoxysilylalkyl aspartic acid ester end-capped polyurethane prepolymers and sealant compositions made therefrom.
Accordingly, there is a need for a silane-based end-capper to create silylated polyurethane resins that have lower VOC admissions. There is also a need for a silane-based adhesion promoter to create sealants, adhesives and coatings that have lower VOC admissions.